Reaction kinetics in the system Y2O3/Al2O3 - A solid state reaction forming multiple product phases investigated by using thin film techniques

The kinetics of the heterogeneous solid state reaction between Al2O3 and Y2O3 is investigated by using thin film techniques. Y2O3 films are grown by means of pulsed laser deposition (PLD) on single crystalline alumina substrates with a (0001) orientation. The solid state reactions are performed at a temperature of 1400 degrees C (1673 K). The cross sections of the reacted samples were investigated by means of SEM and XRD and exhibit a sequence of three product layers, Y3Al5O12 (YAG), YAlO3 (YAP) and Y4Al2O9 (YAM). The simultaneous growth of the product layers is controlled by a diffusional kinetics and the thickness increases are coupled to each other. According to Wagner and Schmalzried, one has to distinguish between rate constants of the first kind ("practical" Tammann constant), in the case of simultanous and coupled growth of multiple product phases, and rate constants of the second kind ("true" Tammann constant), in the case of the uncoupled and only growth of one product phase in equilibrium with the adjacent phases. The growth kinetics for a solid reaction forming three product phases (layer) is analysed in detail using linear transport theory. For the formation of YAG, YAP and YAM the rate constants of the second kind are determined from the experimental data and those of the first kind are calculated and compared to the available literature data. Based on these considerations, a detailed overview about the phase formation kinetics in the temperature range of 1200-1400 degrees C can be given for the first time. The (Nernst-Planck coupled) cation conductivities are calculated.